For certain applications, it is necessary to derive a relatively constant output power from a microwave load in each of a plurality of relatively widely spaced amplitude ranges. The load is likely to have a sloping amplitude versus frequency response over a band that is driven by a microwave source. In one particular application of this nature, a microwave transmitter drives an antenna so that the antenna has an equivalent isotropic radiated power (EIRP, a parameter defined by the product of antenna gain and radiated power) versus frequency characteristic over approximately a 10% band width; for example, from approximately 5.0 to 5.5 GHz. In addition, the output power from the antenna must be maintained level in each of four steps, displaced by 10 db, over a 30 db range.
Maintaining the EIRP of the antenna constant to meet these requirements is difficult because control elements in a feedback path that monitors the power coupled to the antenna and which controls the power coupled between a substantially constant amplitude microwave source and the antenna load generally do not include readily available components that are capable of handling input signals over a 30 db level. In particular, to control the amplitude of the power derived from the antenna in each of the power ranges, it is desirable to employ a linear, crystal amplitude detector in the feedback path. The generally available detectors of this type usually have linear input-output characteristics only over a relatively narrow range of input amplitudes. Generally available linear amplitude detectors are incapable of providing a linear output response over a 30 db range of input signals.